I would like to see this schematic that Don mentions; I look at
ettus.com but all I could find is the 1-page basic description of the
daughterboard. Is there something more detailed that shows the layout of
the 8132 as well as the ADC?

thanks,
eric

p.s.- the USRP performed beautifully for my demo, suitable impressing
what might prove to be the next generation of aerospace engineers!

On Fri, 22 Jun 2007, Matt Ettus wrote:

Don Ward wrote:

That's not what I get. With the input open, I measure 108 mV on the
SMA connector and a DC component from the ADC (detected using the RSSI
register, to avoid the DC offset filter in the FPGA). If I short the
input to ground, I get a negative DC component from the ADC. To get
zero from the ADC I need to ground the input through a 50 ohm
(approximately) resistance; in this case I measure 62 mV at the input
to the LFRX board. My conclusion (confirmed by inspection of the
schematic) is that the LFRX needs to be driven by a source resistance
of 50 ohms *at DC* to be correctly biased.

I am sorry. You guys are both right.
If you connect a 50 ohm resistor across the SMA, you do indeed get 62mV
at the connector, but 0V at the ADC, which is what we really care about.

If you leave the SMA open, you do get a small voltage being read at
the ADC.

So yes, the LFRX does have DC bias on its input, but as long as you
drive it with a 50 ohm load, you get the right answer at the ADC. The
LFRX wasn't designed for measuring DC voltages, although it can as long
as you have a 50 ohm source. And if your source is not 50 ohms, you can
do the calculations necessary to scale the gain.
Also, please note that the amp is inverting, so if you put 1V through 50
ohms into the SMA, the ADC will tell you that it is -1V.
Matt